This invention relates to welding zinc-coated steel sheet using a high energy density beam, such as a laser beam or the like. More particularly, this invention relates to a method for laser welding an assembly comprising at least one zinc-coated steel sheet, wherein, prior to the welding operation, the zinc coating is selectively removed from surfaces in the weld-affected zone and replaced by a nickel coating to reduce weld porosity and provide corrosion protection.
In a typical laser welding operation to join steel sheet members, the members are assembled with faying surfaces in contact, and an external surface opposite the faying surfaces is scanned by a laser beam. The laser beam cuts a keyhole in the members, surrounded by molten metal. After the beam passes, the molten metal fills the keyhole and fuses to form the weld. The laser produces intense heating that permits the weld to be completed in a very short time, on the order of a fraction of a second for a spot weld. In a related laser cladding process, a surface is scanned while depositing a powdered metal, whereupon the powdered metal melts and coalesces to form a coating. An advantage of laser processing is that effects of heating may be limited to a narrow zone immediately about the scanned surface.
Galvanized steel sheet comprises a thin metallic zinc coating for corrosion protection. Zinc has sacrificial properties when applied to steel that make it particularly effective for combating corrosion. However, a major problem is encountered when attempting to weld galvanized steel sheet because the zinc vaporizes at steel fusion temperatures. Zinc vaporization is particularly violent during laser welding because of the intense heating. In addition, because of the rapid rate at which the weld is formed, zinc vapors at the faying surfaces create porosity in the weld nugget that reduces the integrity of the weld. Zinc vaporizing from external surfaces exposes the underlying steel to corrosion.
Accordingly, it is an object of this invention to provide an improved method for welding a zinc-coated thin steel member using a high energy density beam, such as a laser beam, which method includes treating the member prior to welding to replace the zinc coating on selected surface regions with a coating composed of a metal that does not vaporize appreciably at steel fusion temperatures. This coating substitution is effectuated by a single cladding operation and confined to a narrow region immediately about the intended weld. During welding, the substitute coating fuses with the steel to form a sound nugget, in marked contrast to the high porosity produced by violent zinc vaporization. In one aspect, the welded product may be formed between readily available galvanized steel sheet. The zinc coating substantially covers the product to provide preferred corrosion protection, while the substitute coating provides protection at the weld joint.